In its simplest form a rebreather is a device that enables a person to retain and reuse some or all of his or her expired breath. Even with physical exertion, a person uses only a fraction of the oxygen that is inhaled. A rebreather recirculates unused oxygen in the system and replenishes the oxygen as it is used by the wearer. This allows a very small tank of oxygen to last much longer than is possible using traditional SCUBA (Self Contained Underwater Breathing Apparatus) gear. The three main components of typical rebreather systems are gas supply/oxygen control, counterlung, and carbon dioxide removal system.
A rebreather has a carbon dioxide removal system that maintains CO.sub.2 pressures at a safe level. This is relatively easy to do, and is accomplished by passing exhaled gases through a canister filled with a chemical adsorbent, such as soda lime. Several manufacturers make these adsorbents and use their own special mixes. For example, SODASORB.RTM., manufactured by W. R. Grace & Co., consists of a mixture of sodium hydroxide, calcium hydroxide and potassium hydroxide. Other adsorbents, such as lithium hydroxide, can be used to offer improved cold water performance.
Adsorbents are typically in the form of small granules 0.04 to 0.25 inches (1.0 to 6.5 mm) in diameter, placed in a canister through which exhaled gases are passed. Smaller granules allow more surface area per unit weight, but because the person must "breathe" through this canister without too much resistance, larger adsorbent particles are employed so as to allow gas flow around these granules, and through the canister with a relatively low pressure drop. Thus, one of the limitations of current adsorbent canisters is the relatively large adsorbent particle size necessary to get low pressure drops and in turn, ease of breathing.
The current method for filling CO.sub.2 canisters is by pouring the granules into the canister, and then tapping on the canister to get the granules to settle, leaving enough room for a foam liner, which is commonly later compressed, to prevent granule migration. If too much tapping or compression is administered, the granules will particulate or "dust" as they grind together. If not enough settling of the granules has occurred, the granules may shift during diving and cause "channeling" of expired gases through areas of the canister bed that have fewer granules. Thus, depending upon the granule loading technique from person to person, different operational efficiencies can be achieved, with reduced efficiencies occurring due to "channeling" or dusting. Additionally, as is explained below, highly dangerous conditions can occur if loading is not properly performed. Even without these problems, granular adsorbents have to be loaded on-site and are difficult and time consuming to pour into the canister, especially when on the deck of a boat in rough water.
One of the early problems encountered with adsorbent canisters was that breathing in small particles of these adsorbents damaged the user's lungs and throat. Thus, today's adsorbent granules may have non-adsorbing components designed to minimize friability and particulation. This problem can be solved through the use of hydrophobic inlet and outlet filters on the canisters that filter out any airborne particles. The drawback of using these filters however is the increased breathing resistance. By increasing the filter's surface area, the pressure drop can be minimized, but only at the expense of a larger, space consuming filter, such as a pleated filter element. Thus another limitation of existing canisters is the tradeoff between safety, breathing resistance, and size.
Another more serious problem with rebreather systems is that of flooding the rebreather with water by removal of the mouthpiece, or by failure of other systems. If this occurs, water can get into the adsorbent canister, pickup adsorbent particles, and flow into the user's mouth. In fact, it is not even necessary for a system failure to occur to precipitate this dangerous event. Plenty of water from condensation is present in the canister in normal use. This water can mix with adsorbents, and the mixture can work its way around the breathing loop, and into the user's mouth. This occurrence is referred to as "caustic cocktail" in the industry and is especially dangerous when using lithium based adsorbent materials. The use of check valves and water traps can minimize the risk of caustic cocktail. However, these improved systems, while being safer, have the disadvantage of being harder to breathe through and being bulkier. Hydrophobic inlet and outlet filters could also prevent this caustic from exiting the adsorbent canister, but again at the expense of a higher breathing resistance or a bulkier system. Additionally, a hydrophobic inlet filter that prevents water from entering the canister also has the problem of accumulating water, which prevents air from flowing into the canister. Thus an operational problem with this solution to the caustic cocktail, is that just when you are relying on the filter to prevent water from entering the canister, you also prevent air from entering the canister, and the system is rendered useless, and the dive must be aborted.
U.S. Pat. No. 5,165,399 to Hochberg, discloses a CO.sub.2 absorption means in which an adsorbent is intimately admixed with fibrous material and formed into a sheet. The sheet is bonded to a CO.sub.2 permeable fabric layer to improve the overall strength. The technique disclosed allows CO.sub.2 absorption sheets to be produced that are less likely to particulate compared to granular adsorbent systems. The deficiency of this technique is that additional support materials, such as wovens or non-wovens, must be used for structural integrity. Another more important deficiency with this technique is that it does not produce an adsorbent that is designed to be exposed directly to liquid water.
It is accordingly a purpose of the present invention to provide a rebreathing CO.sub.2 adsorbent system that offers little or no risk of inhalation of adsorbent.
It is also a purpose of the present invention to provide a canister that can be flooded without exposing the adsorbents to water, while still allowing the flow of breathing gases.
It is yet another purpose of the present invention to provide a CO.sub.2 adsorbent system that offers all of the aforementioned advantages without a penalty of increased pressure drop.
It is also a purpose of the present invention to provide a rebreather adsorbent system that is easier to load, and less messy than current granular systems.
It is also a purpose of the present invention to maximize the duration that the canister adsorbs carbon dioxide.
These and other purposes of the present invention will become evident from review of the following specification.